Thermoplastic (e.g., polyethylene, polypropylene, etc.) containers and covers are well known in the art and have been used for many years in the battery industry to house the battery's cell elements. The covers have been sealed to the battery containers in a number of ways with the view toward preventing electrolyte leakage from the batteries, and, in some cases, maintaining superambient pressure in the container and preventing ambient air incursion into the battery. A popular technique for sealing a thermoplastic cover to thermoplastic container is a butt-type heat seal made by simultaneously pressing the top edge of the container walls and a corresponding mating surface on the cover against a heated aluminum platen to melt the plastic thereat and, after removing the heated platen, pressing the melted regions of the cover and container together to form a butt weld therebetween. Hayward et al U.S. Pat. No. 441,448 and Kano et al U.S. Pat. No. 3,778,314 disclose variations of this process. This technique is simple and inexpensive but typically produces seals which reliably can only withstand internal pressures of less than about 3 psig. Another technique for sealing covers to containers is to provide a channel around the periphery of the cover, filling the channel with a suitable adhesive (e.g., epoxy resin), embedding the upper edge of the container walls in the sealant and allowing the sealant to cure or otherwise harden. This is a more complicated and costly technique of the aforesaid heat seal process. Another technique, similar to the aforesaid epoxy technique, places a strip of metal particle filled thermoplastic in the cover channel along with the upper edge of the container wall. Thereafter the seal area is bombarded with microwaves to heat the metal particles, fuse the plastic matrix thereabout and bond the cover and the container. Such a technique is shown in Stutzbach et al U.S. Pat. No. 4,460,663 and "Welding Process Seals Deal in Battery Manufacture", Plastics Design Forum, Sep./Oct. 1988, pages 36 and 37. Like the aforesaid epoxy process, this latter technique is more complicated and more expensive than the simpler heat seal technique.
So-called "gas recombination" batteries, operating on the oxygen cycle, consume the internally generated gases within the battery itself rather than exhausting them to the atmosphere. Batteries of this type are sealed and contain pressure relief valves for venting the gases to the atmosphere only after pressure within the container exceeds a predetermined value. Generally speaking, higher internal pressures result in more effective gas recombination with internal pressures of at least about five (5) psig being desirable. The operating pressure of batteries has heretofore been limited to some extent by the ability of the container-cover seal to withstand these higher internal pressures. It would be desirable therefore to have a heat seal between the container and the cover which is both simple and economical to make, and which is capable of withstanding higher internal pressures than are possible with butt-type heat seals alone.
It is an object of the present invention to provide an improved, easily made heat seal between a thermoplastic container and cover and a process for making same, which seal is capable of withstanding internal container pressures greater than about 5 psig without the need for extraneous sealant or filler material at the joint between the container and cover.